This invention relates to a method and apparatus for adding texturing highlights to a video signal.
Among the video effects that can be applied to an array of sample values representing a luminance field to produce an enhanced array which represents a somewhat different luminance field is an effect known as "embossing". Edge information is extracted from an image and is used to add luminance along edges of one polarity and subtract luminance along edges of the opposite polarity. The term "polarity" as applied to an edge is intended to be understood as referring to the sign of the change in luminance across the edge when the edge is traversed in a particular direction. If the luminance increases, the edge is of positive polarity, and if the luminance decreases the edge is of negative polarity. Clearly, an edge that is of positive polarity when traversed in one direction is of negative polarity when traversed in the opposite direction. When luminance is added and subtracted along edges in the original luminance field, the areas of increased and reduced luminance appear to the eye as highlights and shadows which provide three-dimensional cues for the eye and achieve an embossed texture appearance.
The video effect of embossing is currently performed by applying the luminance field array to a 3.times.3 high pass filter kernel. All coefficients of the kernel are 0 except two which are symmetrically disposed about the central element. These two coefficients are of equal and opposite magnitude. The kernel accentuates edges that extend transversely of the line between the two non-zero coefficients. For example, if the positive coefficient is at the upper left corner of the kernel and the negative coefficient is at the lower right corner, the kernel accentuates the edges that extend diagonally from the lower left corner of the luminance field to the upper right corner. If the resulting edge information array is combined with the original luminance field array, edges that are of positive polarity when the image array is traversed from the upper left corner to the lower right corner are highlighted and edges of negative polarity are shadowed, so that it appears that the scene represented by the enhanced luminance field is three-dimensional and is illuminated by a light source above and to the left of the viewer. Other permutations of this kernel allow simulation of a light source at other positions. However, the above-described 3.times.3 kernel cannot be used to simulate illumination other than along an axis which is horizontal or vertical or is at 45.degree. to the horizontal and vertical axes. The effect of illumination at an arbitrary angle cannot be simulated by this method.